BORIS Johnson has been rebuffed by Brussels after demanding major changes in a new Brexit deal.

The Prime Minister set out his call for the backstop – the contingency plan to avoid a hard border with Ireland – to be scrapped from the divorce deal ahead of the October 31 Brexit deadline.

But European Council president Donald Tusk defended the measure and warned that those seeking to replace it would risk a return to a hard border on the island of Ireland.

Tusk tweeted: "The backstop is an insurance to avoid a hard border on the island of Ireland unless and until an alternative is found.

"Those against the backstop and not proposing realistic alternatives in fact support reestablishing a border. Even if they do not admit it."

Johnson had written to Tusk night outlining his opposition to what he called the "anti-democratic" Northern Ireland backstop.

READ MORE: The five most ridiculous lines from Boris Johnson's letter to the EU

In the letter, Johnson said while he wants the UK to leave the EU with a deal, he could not support any withdrawal agreement that "locks the UK, potentially indefinitely, into an international treaty which will bind us into a customs union and which applies large areas of single market legislation in Northern Ireland".

However, Commission spokeswoman Natasha Bertaud said "the letter does not provide a legal operational solution to prevent the return of a hard border on the island of Ireland".

She added: "It does not set out what any alternative arrangements could be and in fact it recognises that there is no guarantee that such arrangements will be in place by the end of the transitional period."

Ireland's premier Leo Varadkar had already rejected Johnson's call to reopen the Withdrawal Agreement during a telephone conversation with the Prime Minister on Monday.